Method and apparatus for displacing liquor from a slurry of particulate solid material

ABSTRACT

A slurry is subjected to a filtration and washing process in order to separate the liquor from the solids. The filtration and washing process is carried out in a housing (101) enclosing an inert gaseous atmosphere which also passes through the filter cake and may be recycled, after treatment, to remove contaminants therefrom. To prevent recontamination of the filter cake as a result of the recycled gas entraining contaminant vapors within the housing (101), the gas is introduced into the housing (101) at a downstream location (152a, 166) relative to the direction of travel of the filter cake on a movable filter medium (100) and in such a way as to develop a contaminant concentration gradient whereby the cleanest cake is contacted by gas having the lowest contaminant level.

FIELD OF INVENTION

This invention relates to a filtration process in which a gaseousatmosphere is to be maintained over a solids material from which aliquor is to be displaced.

For instance, the gaseous atmosphere may be required in order to excludeat least in part oxygen from a system in which filtration takes placeand, in this event, the gaseous atmosphere may typically comprisenitrogen or other inert gas. By "gas", we include media which, at roomtemperature and pressure tend to be in a phase other than the gas phase,but which under the conditions at which the filtration process isperformed exist in the vapour phase, eg steam.

The invention finds specific application in, for example, those stagesof terephthalic acid production involving separation of terephthalicacid crystals from a liquor in which the crystals are slurried.

BACKGROUND OF THE INVENTION

In the production of terephthalic acid, slurry streams containingterephthalic acid crystals may arise at one or more stages in theprocess and the nature of the liquor in which the crystals are slurriedmay vary. For instance, where the crude terephthalic acid is initiallyproduced by the liquid phase oxidation of paraxylene in the presence ofa carboxylic acid, such as acetic acid, and a suitable catalyst system(typically cobalt, manganese and bromine compounds), the crudeterephthalic acid is withdrawn from the reactor as a slurry ofterephthalic acid crystals in liquor comprising acetic acid and waterwith dissolved impurities, including terephthalic acid precursors suchas 4-carboxybenzaldehyde (4-CBA) and paratoluic acid. If the crudeterephthalic acid is thereafter purified by hydrogenation of an aqueoussolution thereof (possibly preceded by a further stage of oxidation ofthe crude terephthalic acid in aqueous solution to convert 4-CBA toterephthalic acid), a slurry of purified terephthalic acid in aqueousliquor results where the aqueous liquor may have dissolved thereinimpurities such as paratoluic acid. In both cases, the terephthalicacid, either crude or purified, has to be freed of the correspondingliquor in a highly efficient manner.

A suitable technique in this latter respect is afforded by a combinedfiltration and washing system such as that disclosed in our priorpublished EP-A-502628 and copending International patent application No.PCT/GB 93/01019, the entire disclosures of which are incorporated hereinby reference. In the systems disclosed in these prior applications, thedisplacement of liquor from wet filter cake comprising terephthalic acid(crude or after purification) is effected by transporting the filtercake on a belt filter through a washing zone in which an aqueous washliquid is supplied to the filter cake in a number of stages at differentpoints along the path of travel of the belt. The wash liquid displacesthe liquor from the filter cake and the liquor together with the washliquid passes through the filter material forming the belt.

In practice, it is necessary to establish an atmosphere of inert gasover the filter cake, for instance to exclude or control the level ofoxygen present and/or to assist in drying of the filter cake. This inertgas may pass through the filter material in the washing zone and/or in azone or zones downstream of the washing zone. Additional gas must beintroduced in order to maintain the gaseous atmosphere. The inert gasmay be nitrogen for instance, although in the case of the filter/washingsystem employed in the purification stage of the process, the inert gasmay with advantage comprise steam for reasons disclosed in ourco-pending International patent application No. PCT/GB 93/01033 (theentire contents of which are incorporated herein by reference).

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved form of filtrationand washing process and system.

According to one aspect of the present invention there is provided amethod of displacing liquor from a solids material comprising:

forming the solids material into a layer on a movable filter medium;

transporting the layer by means of the filter medium through a washingzone in which the layer is contacted along the path of movement thereofwith a wash medium, the wash medium serving to displace liquor from thelayer and passing through the filter medium;

establishing over said layer a gaseous atmosphere from which gas passesthrough the layer; and

supplying gas to the gaseous atmosphere so as to produce a concentrationgradient within the gaseous atmosphere such that the liquor content ofthe gas passing through said layer increases in a directioncounter-current to the direction of travel of said layer.

The concentration gradient may be substantially continuous lengthwise ofthe direction of travel of said layer or it may vary in a stepwisemanner.

The concentration gradient may be produced by effecting flow of said gasin countercurrent relation with the direction of travel of the layer ofsolids material whereby liquor evaporating from the layer of solidsmaterial in said zone and upstream of said zone is substantiallyprevented from passing downstream of said zone.

Alternatively the concentration gradient may be produced by dividing theregion in which the gaseous atmosphere is established into a number ofzones lengthwise of the direction of travel of said layer, such that thelevel of contamination of the gas with said liquor differs from one zoneto another. Thus, for instance, the gas to be introduced into one zoneadjacent the downstream end of the path of travel of said layer may besubjected to more intensive clean-up than gas to be introduced into oneor more upstream zones. The division of gaseous atmosphere region intosubstantially isolated zones may be effected by means of suitablepartitioning devices such as hood arrangements each arranged insuperimposed relation with a corresponding section of the path of travelof said layer and to which gas having a level of contaminationsubstantially less than that of the layer is supplied.

The gas supplied to the gaseous atmosphere is preferably at least inpart (usually at least a major part) gas recycled from the filtrate sideof the filter medium after treatment to reduce the level ofcontamination with liquor. The recycled gas may be supplemented withclean make-up gas as necessary. However, we do not exclude thepossibility of supplying to the gaseous atmosphere fresh gas rather thanrecycled gas. In this event, where the gas recovered from the filtrateside of the filter medium it may be used elsewhere in the process,particularly where the gas employed is steam.

According to a second aspect of the present invention there is provideda method of displacing liquor from a solids material comprising:

forming the solids material into a layer on a movable filter medium;

transporting the layer by means of the filter medium through a washingzone in which the layer is contacted along the path of movement thereofwith a wash medium while maintaining in a region over the layer agaseous atmosphere, the wash medium serving to displace liquor from thelayer and passing, together with the liquor and gas from saidatmosphere, through the filter medium;

recovering the gas and treating it to eliminate or at least reducecontamination thereof by the liquor; and

reintroducing the treated gas into said atmosphere at a location suchthat the gas passing through the layer at a location downstream of, orwithin a downstream section of, the washing zone is less contaminatedwith liquor than that passing through the layer at a location upstreamof, or within an upstream section of, the washing zone.

Conveniently the washing zone comprises a series of washing stagesarranged in succession along the path of travel of the filter medium andthe washing medium may be passed through the washing stages incounter-current relation with the direction of movement of the filtermedium.

In one form of the invention, the recovered gas is separated into twostreams which are treated to differing extents such that one stream hasa lower contaminant level than the other, and in which the two streamsare reintroduced into said atmosphere at different locations, said onestream being reintroduced at a location downstream of the location atwhich the other stream is reintroduced. The recovered gas may besubjected to cooling prior to being split into two streams so thatcontaminants in the vapour phase are condensed and thereby separatedfrom the gas.

According to a further aspect of the invention there is provided asystem for processing a slurry comprising a solids material and aliquor, said system comprising:

means for filtering the solids material to form a layer thereof on amovable filter medium;

a washing zone;

means for driving the filter medium so as to transport the layer throughthe washing zone;

means for applying wash medium to the layer during traverse of thewashing zone, the wash medium serving to displace liquor from the layerand passing through the filter medium; and

means for establishing in said washing zone and/or a zone downstreamthereof a gaseous atmosphere from which gas passes through the layer andthe filter medium, said atmosphere-establishing means being arranged toproduce a concentration gradient such that the extent to which the gaspassing through said layer and the filter medium is contaminated withsaid liquor decreases in the direction of travel of the filter medium.

According to another aspect of the invention there is provided a systemfor processing a slurry comprising a solids material and a liquor, saidsystem comprising:

means for filtering the solids material to form a layer thereof on amovable filter medium;

a washing zone;

means for driving the filter medium so as to transport the layer throughthe washing zone;

means for supplying pressurised gas to the washing zone to establish apressurised gaseous atmosphere on that side of filter medium on whichthe layer is formed; and

means for applying wash medium to the layer during traverse of thewashing zone, the wash medium serving to displace liquor from the layerand passing, together with the liquor and gas from said atmosphere,through the filter medium;

the pressurised gas supplying means being arranged to create a gas flowin countercurrent relation with the direction of travel of the layer ofsolids material whereby liquor evaporating from the layer of solidsmaterial in said zone and upstream of said zone is substantiallyprevented from passing downstream of said zone.

DETAILED DESCRIPTION OF THE INVENTION

In the context of terephthalic acid production, the filtration andwashing may be applied to the separation of terephthalic acid crystalsfrom a mother liquor comprising solvent employed either in an oxidationreaction for the production of terephthalic acid or in an hydrogenationreaction to purify crude terephthalic acid. In the former case, thesolvent is usually an aliphatic carboxylic acid such as acetic acid andin the latter case the solvent is usually water. In both instances, thepressurised gas may comprise nitrogen; however, in the latter case, itis advantageously steam as the filtration process can be carried out insuch a way that chilling of the filter cake is reduced or substantiallyeliminated as disclosed in our copending International PatentApplication No. PCT/GB 93/01019.

The filter medium is suitably a metal gauze, or a cloth comprising aplastics material such as polyester, polypropylene, polyetheretherketoneand the like in which case the cloth may be woven from filaments of thepolymeric fibre using a weave suitable for the specific filtrationapplication. Filter media woven from polyetheretherketone isparticularly suitable in the production, and purification, ofterephthalic acid, especially in the filtration of crude terephthalicacid in order to separate the same from acetic acid containing motherliquor. The filter medium my be configured as a loop which may becontinuous (as in a belt filter of the Pannevis type) or may comprise aseries of discrete sections (as in a rotary vacuum filter or a rotarypressure drum filter). In each case, the filter medium may be movedcontinuously or intermittently to convey the solids material through thewashing zone. Such rotary vacuum filters and pressure drum filters ofthis type are described in the literature, see for example Pages 252-254of the textbook "Industrial Filtration of Liquids" by D B Purchas (1967edition, Chemical & Process Engineering Series published by LeonardHill).

Where the filter system comprises a pressure drum filter, it willtypically be of the type in which the wash liquor is supplied to thehousing of the drum under pressure and in which the washed filter cakeis subsequently dried by passage of gas through the filter cake as thedrum continues to rotate to advance the filter cake from the washingzone towards a discharge point. In such an arrangement, the gas fordrying of the filter cake is supplied in such a way as to establish overthe filter cake a gaseous atmosphere with a concentration gradient inthe manner referred to herein, ie so that the gas passing through thefilter cake at locations near to the point of discharge of the filtercake from the drum is less contaminated than that passing through thefilter cake at upstream locations in a direction towards the washingzone(s) of the drum. The variation in level of contamination of the gasmay be progressive or it may be discrete (eg by partitioning the gasdrying region into zones and introducing gas with differing levels ofcontamination into the zones, as previously referred to).

The washing zone suitably comprises a succession of stages in which, ineach stage (other than the last), the incoming wash medium passesthrough the solids end the filter medium in counter-current relation tothe direction of travel of the layer of solids material. In the laststage the wash medium is preferably fresh incoming water.

In a zone upstream of the washing zone, the solids material may besubjected to an initial filtration stage to separate a major part of theliquor from the solids material, the residual liquor content of the wetsolids material thereafter being largely removed in the washing zone.

In a third zone downstream of the washing zone, the layer of solidsmaterial may be ejected, scraped off or otherwise removed from thefilter medium. Where the solids material tends to adhere at least inpart to the filter medium, the preferred method is to wash the layer ofsolids material off with an aqueous medium, preferably substantiallypure water, which may be in the form of jets of liquid at the downstreamend of the path of travel of the filter medium.

In the case of a continuous band, it is desirable to provide suitablemeans to pass liquid for example water or alkaline solution, through thereturning part of the band to wash off downwardly facing adheringdeposits into a receiver.

Desirably, there is a pressure differential across the movable filtermedium, with the side of the filter medium on which the slurry isdeposited being at a higher pressure than the other side of the filtermedium. Suitably the differential pressure is at least 0.05 bar and, inthe case of terephthalic acid production, no more than the pressure atwhich the oxidation or purification step (as the case may be) is carriedout, for example 30 bar in the case of the oxidation reaction.

Preferably the pressure differential is 0.1 to 15 bar, more preferably,0.2 to 7 bar and especially 0.3 to 3 bar, for example 0.6 bar.

Suitably the higher pressure side of the band is at substantially thesame pressure or a higher pressure than the preceding step in theprocess, for example a crystallisation step or the oxidation step.

In the context of terephthalic acid production by liquid phase oxidationof paraxylene, the slurry of terephthalic acid in acetic acid issuitably deposited on the movable filter medium at a temperature of atleast 60° C. and preferably 70° to 200° C., especially 80° to 150° C.Suitably the slurry is deposited in such a way that the saturationpressure of the feed is less than the absolute pressure on the lower(downstream) side of the filter medium.

Deposition of the slurry at elevated temperature is advantageous asimproved filtration is possible due to the reaction medium being lessviscous at elevated temperature. Furthermore there is lessco-crystallisation of impurities for example 4-carboxybenzaldehyde, withthe terephthalic acid product at elevated temperature.

The other individual steps of the terephthalic acid production processcan be carried out conventionally. The liquid reaction medium normallycomprises a catalyst, for example a cobalt/manganese/bromide catalystsystem which is soluble in the reaction medium. Suitably the oxidationis carried out in the presence of an oxygen source for example air, at apressure of 5 to 30 bars absolute, and preferably an oxygenconcentration of 0 to 8% by volume in the gas leaving the reactor and ata temperature of 150° to 250° C. It is suitably a continuous process,and is preferably carried out in a stirred reactor. The reaction isexothermic and the heat of the reaction may conveniently be removed byevaporation of water and acetic acid from the reaction medium.

Suitably the crude terephthalic acid product obtained by liquid phaseoxidation of paraxylene followed by filtration and washing of the crudeproduct in accordance with the method according to the present inventionis purified by a process which comprises:

dissolving the crude terephthalic acid in aqueous medium to produce asolution comprising terephthalic acid;

contacting, under reducing conditions, the said solution with hydrogenand a heterogeneous catalyst for the reduction of at least someimpurities;

cooling the solution to precipitate solid purified terephthalic acidproduct; and

recovering the said product from the solution.

If desired, in order to reduce the level of impurities present in thecrude terephthalic acid, especially 4-CBA, the crude terephthalic acidmay be dissolved in an aqueous medium and subjected to oxidationtreatment using gaseous oxygen or other oxidising agents (notnecessarily in the gaseous phase) to convert at least part of the 4-CBAimpurity content to terephthalic acid.

Suitably the heterogeneous catalyst employed in the purification of thecrude terephthalic acid product may be a supported noble metal catalyst,for example platinum and/or preferably palladium on an inert, forexample carbon, support. The reduction is suitably carried out bypassing the aqueous solution comprising terephthalic acid andimpurities, for example 4-carboxybenzaldehyde, through a flooded bed ofcatalyst as a temperature of 250° to 350° C. in the presence ofhydrogen. The solution suitably comprises 20 to 50% by weight ofterephthalic acid.

The solution after reduction is suitably cooled to a temperature in therange 100° to 250° C. to separate pure terephthalic acid product fromthe solution. This solution is preferably subsequently cooled to atemperature in the range 15° C. to 100° C. or evaporated to produce aless pure precipitate and a mother liquor. The less pure precipitate issuitably separated from the mother liquor. The mother liquor from thisseparation my be recycled directly or indirectly to distillation and/orbe used as the second aqueous medium to reslurry the crude terephthalicacid. If desired the less pure precipitate may be recycled to theoxidation step.

Alternatively, if purification is not employed, the terephthalic acidfollowing filtration and washing may be removed (without necessarilyslurrying it in aqueous medium) and used in polyester production,directly in some instances--eg production of polyethylene terephthalatearticles such as bottles. This is made feasible by virtue of the reducedlevels of contamination that can be achieved by means of an integratedsolids-liquid separation and countercurrent solids washing process (egby means of a belt filter system) combined with use of the inerting gasto suppress downstream contamination of the washed solids by liquorvapours.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to the accompanying drawings, in which:

FIG. 1 is a flowsheet relating to a process for the production ofpurified terephthalic acid and to which the present invention can beapplied;

FIG. 2 is a schematic representation of a filtration and washing systemwhich operates in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 1, a reactor A for effecting the liquid phaseoxidation of paraxylene is supplied with paraxylene and acetic acidcontaining a dissolved catalyst system comprising cobalt, manganese andbromine ions (via line 1) and with air via line 2. Product is withdrawnfrom the reactor A via line 3 and is passed to crystallisation sectionB. The temperature within the reactor A is regulated by withdrawing amixture of acetic acid and water vapour from the reactor and passing themixture to a condensing system C via line 4. Most of the condensate isreturned to the reactor A via line 5 and non-condensibles may be ventedvia line 6. To control the water content within the reactor A, part ofthe condensate is removed from the condensing system via line 7 andpassed to the distillation column D via line 9.

In the crystallisation section B the temperature is dropped toapproximately 80° C. to 150° C. and the slurry containing crystallineterephthalic acid in mother liquor (mainly acetic acid) thereby producedis passed to a filtration stage E. Mother liquor recovered from thefiltration stage is returned in part to the reactor A via lines 10 and10a. Acetic acid may be recovered from crystallisation section B viastreams 8 and 9 to the distillation column D and/or via streams 8 and10a to the reactor A. The solids material recovered from filtrationstage is transferred to reslurry stage F via line 15 where the recoveredterephthalic acid crystals are reslurried with water which may comprisewater derived from elsewhere in the process, eg from distillation columnD via stream 14, recycled mother liquor via stream 18, recycled motherliquor via stream 16 and/or demineralised water via stream 17.

Referring to FIG. 2, the filtration stage E and reslurry stage F areintegrated into a unit in which the terephthalic acid crystals are alsowashed to reduce or eliminate acetic acid contamination thereof. Theunit shown in FIG. 2 comprises belt filter equipment such as a Pannevisfilter of the form generally described in Filtration and Separation(Page 176 et seq, Mar./Apr. 1979). The belt filter equipment comprises agenerally horizontally disposed filter belt 100 which runs over twodrums 102 and a series of rollers (not shown), suitable drive meansbeing provided for driving the filter belt 100 so that the upper run 104thereof travels from left to right as seen in FIG. 2. The filter belt100 may comprise a cloth woven from polyetheretherketone monofilamentsusing a suitable weave.

The filter band 100 is enclosed in vapour tight housing 101 one end ofwhich, located adjacent the downstream end of the belt filter upper run104, is connected to the reslurry vessel F by a discharge chute 103. Theinterior of the housing is pressurised with a suitable gas (as discussedhereinafter), the pressure within the housing typically being in excessof 1 bara, eg 3 to 15 bara. Beneath the upper run, a suction tray unitis located which, in the illustrated embodiment, comprises four trays106. The trays 106 are coupled together for movement as a unit in adirection parallel to the direction of travel of the upper belt run 104,the tray unit being movable in a reciprocating fashion between the drums102 such that it can travel with the belt from one position adjacent theleft hand drum to a position adjacent the right hand drum and thenreturn to the first position. During travel of the tray unit 106 fromleft to right, suction is applied to draw liquid through the upper run104 of the belt and during the reverse travel, suction is terminated.The pressure differential across the filter belt (ie. between the regionabove the upper run 104 and the interior of the suction tray unit 106)is usually at least 0.03 bar and typically of the order of 0.6 bar.

Dotted lines show the locations of three zones Zl, Z2 and Z3. In thefirst zone Z1 on the left, a slurry of terephthalic acid and acetic acidtogether with any dissolved catalyst is introduced via line 108 onto theupper run 104 of the filter belt and acetic acid is drawn through thebelt into suction tray 106 from which it is removed via line 110,thereby leaving a filter cake (typically at least one inch thick) ofterephthalic acid on the upper run 104. At this stage, the filter cakewill contain residual contaminants, particularly residual acetic acid.

In the second zone Z2, a wash medium (typically water) is introduced vialine 112 and passes through the belt run 104 in countercurrent fashionrelative to the direction of travel of the filter cake. The wash mediumis initially discharged onto the filter cake via outlet 114 so as form alayer over the filter cake and is effective with the assistance of thegas pressure within the housing 101 to displace contaminants such asacetic acid, if still present, through the belt run 104 into the suctiontray unit, the displaced acetic acid and wash medium being drawn throughthe upper run 104 and into the tray unit 106 from which the resultingfiltrate (wash medium and residual acetic acid) is withdrawn and re-usedfor washing of the filter cake at a position upstream of outlet 114. Inparticular, the filtrate is passed via line 116 to filtrate receiver 118and is pumped by pump 120 via line 122 to outlet 124 upstream of outlet114 so that the filtrate is effective to displace acetic acid and anyother contaminants through the upper run 104 into the suction tray unit106. In the illustrated embodiment, the procedure is repeated againusing liquor supplied via line 126, filtrate receiver 128, pump 130,line 132 and outlet 134. In this manner, the filter cake on the upperrun 104 may be washed with increasingly purer water as it traverses thethree washing stages corresponding to the outlets 134, 124 and 114.

In the third zone Z3, the filter cake is discharged from the filter belt100 and water discharged from nozzles (not shown) onto the filter belt100 serves to dislodge any filter cake tending to adhere to the filterbelt. The discharged filter cake falls into the discharge chute 103 towhich water is fed via line 135, this water being derived from lines 14,16, 17 and/or 18 (FIG. 1). The water employed to dislodge the filtercake from the filter belt and/or to supply the outlet 114 may be derivedat least in part from line 14, 16, 17 and/or 18 (see FIG. 1). Thefiltrate obtained after passage of the wash medium (comprising forexample water and contaminants, particularly acetic acid derived fromthe previous wash stages) from outlet 134 through the filter cake iscollected in suction tray unit 106 and is passed to filtrate via line136 and is passed to mother liquor receiver 138 together with thefiltrate recovered via line 110 from the first zone Z1. Mother liquorrecovered in this way may be returned at least in part via line 140 (andline 10) to the reactor A optionally by first mixing with the freshcatalyst, paraxylene and acetic acid contained in line 1. Any remainingmother liquor and wash liquid is suitably passed to an evaporation stageG via line 12 (FIGS. 1 and 2) in which water and acetic acid vapour isremoved by line 11, condensed and passed to reactor A or optionallypassed to distillation column D and a purge of by products and catalystis withdrawn via stream 13.

From the foregoing, it will be seen that the filtration equipmentdescribed performs the dual role of separating terephthalic acidcrystals from acetic acid rich mother liquor and washing the separatedfilter cake with water to displace residual acetic acid mother liquor,the overall effect being solvent interchange, ie. replacement of theacetic acid solvent by water. Both duties are performed on a singlesubstantially horizontal belt with the washing duty being performed bymeans of a multi-stage countercurrent wash. In order to achieve a highoverall wash efficiency, cross-mixing of liquor between the wash stagesmust be minimised. This is achieved by allowing a small mount of gasbreakthrough between the stages, ie. after all liquor on the surface hasbeen displaced through the filter cake in each stage/zone.

Because of the presence of flammable materials within the oxidationplant, the belt filter system is inerted by the introduction of asuitable gas (typically nitrogen) into the housing 101. During operationof the belt filter, nitrogen drawn through the cake onto the lowerpressure side becomes near saturated with acetic acid/water vapours. Thenitrogen and acetic acid/water vapours enter the receivers 118, 128 and138 and are collected via line 141 and passed to a vapour condenser 142where the nitrogen is substantially freed of the acetic acid/watervapours, the latter being condensed and circulated via knock-out drum144 and line 146 to the mother liquor receiver 138. The nitrogenrecovered in this way is recompressed and returned to the housing 101via line 148, recirculation blower 150 and valve-controlled line 152.Part of the nitrogen circulating this "closed loop" system is bled offvia valve-controlled line 154 in order to regulate the oxygenconcentration in the system and make-up nitrogen is introduced into thehousing via line 155.

Instead of the condenser 142/knock-out drum 144 arrangement describedabove in which condensation of the condensibles is effected by indirectheat exchange in condenser 142, in an alternative arrangement, thesecomponents are replaced by a scrubbing tower in which the nitrogen andacetic acid/water vapours are contacted with a cooled scrubbing liquor(which itself may be derived at least in part from the condensed aceticacid/water vapours) to effect direct, rather than indirect, heatexchange. After contact with the incoming nitrogen and acetic acid/watervapours, the scrubbing liquor and condensed vapours are withdrawn fromthe bottom of the scrubbing tower and may be recirculated back at leastin part to the top of the tower via an indirect heat exchanger to effectcooling of the scrubbing liquor.

It has been determined that whilst a filtration and washing system asdescribed above is particularly effective in displacing mother liquorfrom the filter cake, unaccountably the filtered and washed terephthalicacid may still have an undesirably high residual acetic acid contentdespite being subjected to extensive washing with a countercurrentwashing arrangement, suggesting that the washing efficiency may not beas efficient as expected. Surprisingly, the unexpectedly high level ofresidual acetic acid in the washed terephthalic acid has been found notto be attributable to poor washing efficiency. It transpires that theinerting gas itself is the source of the problem and also terephthalicacid has been found to exhibit a propensity to take up acetic acid fromthe nitrogen; thus, irrespective of the efficiency of acetic acid/watervapour removal from the gas prior to recycling the gas, the manner inwhich the gas is re-introduced into the housing 101 and the manner inwhich it passes through the housing are important factors in determiningthe extent of residual acetic acid contamination in the crudeterephthalic acid recovered from the filtration and washing system.

In zone Z1, where the hot feed slurry enters the housing 101, someevaporation of the acetic acid/water present in the slurry occurs intothe surrounding vapour space. If the recycle gas is returned to thehousing 101 at a location such as that indicated by line 152, then itwill tend to distribute itself around the housing 101 to satisfy theflow requirements through the cake, and in particular it will tend toflow downstream co-current with the direction of travel of the filtercake. As a result, it has been found that the gas tends to sweepevaporated acetic acid/water downstream with potential forrecontamination of the terephthalic acid filter cake with acetic acidespecially in the final washing stages and beyond.

In experimental work, we have established that nitrogen saturated withacetic acid, if drawn through an "acetic acid free" filter cake, leadsto significant contamination of the filter cake with acetic acid. In oneexperiment, 4.8 dm³ of nitrogen gas at 40° C. saturated with acetic acidvapour was drawn through a 40 mm thick cake of terephthalic acid on a100 cm² Buchner funnel over a 10 second period. The initial cake aceticacid content was less than 100 ppm w/w acetic acid; after the gas hadbeen drawn through the cake, the acetic acid content has risen to 0.27%w/w at the top of the cake, falling to 0.14% w/w at the bottom of thecake, demonstrating the surprising propensity of terephthalic acid totake up acetic acid in the gas stream. Very little acetic acid wasdetected in the exit nitrogen stream.

By locating the re-entry point for the recycle nitrogen gas at oradjacent the downstream end of the housing 101 (for instance, asindicated by reference numeral 152a), it is possible to secureredistribution of the gas in such a way that the gas tends to flowcounter-current to the direction of travel of the filter cake upstreamof the location 152a, and hence counter-current to the acetic acid"gradient" in the filter cake. For a relatively low acetic acid contentin the recycle gas, this ensures that acetic acid rich vapours reside atthe upstream end of the housing 101 and are substantially prevented fromflowing co-current with the filter cake, especially into the finalwashing stage of zone Z2.

Even relatively low acetic acid content in the recycle gas can causesignificant contamination of the filter cake in the later wash stage(s)if very high overall washing efficiency is required. In this event, therecycled gas may be subjected to aqueous scrubbing or other techniquefor removing acetic acid before being introduced into the housing at oradjacent the downstream end thereof. Thus, for example, the gas obtainedfrom the knockout drum 144 may be scrubbed before being returned to thehousing 101. However, because it is over the final wash stage(s) wherere-contamination of the filter cake by the gas is especially important,rather than scrub the entire quantity of recycle nitrogen which wouldrequire a scrubber of substantial size, a more cost effective approachis achieved by splitting the recycle nitrogen into two streams. Onestream is returned via line 152a (without scrubbing) and the otherstream is routed via line 156 and valve 158 to a relatively smallscrubber 160 in which the nitrogen gas is contacted in counter-currentfashion with an aqueous scrubbing medium (which may be derived from anyone or more of lines 14, 16, 17 and 18) introduced via line 162 andwithdrawn via line 164. The scrubbed nitrogen (which is substantiallyfreed of acetic acid) is then routed via line 166 to a location withinthe housing 101 adjacent the filter cake discharge end so that it passesthrough the filter cake at least in the final washing stage(s) and/or alocation beyond the latter.

In this manner, the nitrogen with differing acetic acid contents isre-introduced at two locations. The overall effect is to achieve acounter-current sweep of vapour towards zone Z1, with near "acetic acidfree" vapour contacting the "cleanest" cake. For similar reasons, themake-up nitrogen fed into the housing 101 via line 155 enters at thecake discharge end of the housing 101 for example as shown or at alocation such that it is effective to purge the vessel F. A curtain 200(for instance in the form of a suspended flap of flexible material suchas rubber) may be used inside the housing 101 to further assist inpartitioning the "clean" and "dirty" gas sections.

In reslurry vessel F the crystals may be reslurried with water recoveredfrom the distillation column D via stream 14 and/or other water whichmay be recycle mother liquor via stream 18, recycle mother liquor viastream 16 and/or demineralised water via stream 17. The slurry producedin this stage is heated in section H to a temperature of for example250° C. to 350° C. to form a solution which is passed via stream 19 toreactor J in which it is reacted with hydrogen over a fixed bedpalladium catalyst thus reducing impurities in the solution and thenagain crystallised in crystallisation section K from which pure productis separated and dried in stage L which my comprise a filter/washingsystem similar to that described above in relation to FIG. 2, and adrier. In this instance, the washing process is employed primarily todisplace mother liquor comprising water and some dissolved paratoluicacid and other impurities (eg. colour impurities, metals, etc.) from theterephthalic acid and the washing process may not need to be asextensive in which case the counter-current arrangement may be dispensedwith. Thus, for example, in the filtration/washing process for thepurified terephthalic acid the arrangement may be such that the washmedium only makes one pass of the filter cake.

The temperature to which the solution is cooled in the crystallisationsection K and the rapidity of cooling is adjusted to produce theappropriate purity of the desired terephthalic acid product. The pureterephthalic acid product is recovered from stage L and the motherliquor from the separation is passed to recovery stage M in which theliquid is evaporated or further cooled so as to permit the recovery offurther solids which may be passed back to reactor A via stream 20. Instage M the temperature of the liquor may be reduced to 100° C. byflashing steam from it at atmospheric pressure. Such steam may befurther purified for example by distillation and used if desired as washin stage L, used elsewhere in the process or purged. The remainingliquor may be cooled or evaporated further and solids separated from it.The mother liquor recovered from stage M may be in part passed back tothe distillation column D via line 22, in part be returned to thereslurry stage F via stream 16 and in part be purged via stream 21.Preferably if evaporation is used the evaporated water is returned tothe reslurry stage F.

Where the stage L comprises a filtration and washing system similar tothat described in relation to FIG. 2, the inerting gas may again benitrogen; however, it is preferred to employ steam as the inerting gasfor reasons that are disclosed in copending International PatentApplication No. PCT/GB 93 01019. In this event, the filtration/washingsystem would be employed in an analagous manner to separate pureterephthalic acid crystals from dirty mother liquor and wash theseparated filter cake with clean water to displace any residual dirtymother liquor from the cake. In these circumstances, the steam isemployed so as to develop a counter-current sweep of steam so as toensure that any volatiles in the dirty mother liquor are retained at theupstream, slurry feed end of the filter belt.

Although the invention is disclosed herein with reference to a filterbelt type application, it will be appreciated that it may also beapplied to other types of filtration system suitable for carrying outfilter cake washing, eg the invention may be applied to a rotary suctionfilter in which the solids material is transported by a cylindricaldrum-mounted filter medium through filtration and washing stages and inwhich the filtration and washing process is enhanced by a pressurisedgas, with a pressure differential established between opposite sides ofthe filter cake in use. As described in relation to FIG. 2, in such anembodiment the pressurised gas is drawn through the filter cake and thenrecycled, following treatment to eliminate or reduce contaminants, to alocation corresponding to the downstream end of the path of travel ofthe filter cake.

I claim:
 1. A method of displacing liquor from a particulate solidmaterial comprising:forming the particulate solid material into a layeron a movable filter medium; transporting the layer by means of thefilter medium through a washing zone in which the layer is contactedalong the path of movement thereof with a wash medium, the wash mediumserving to displace liquor from the layer and passing through the filtermedium; establishing over said layer a gaseous atmosphere from which gaspasses through the layer; and supplying gas to the gaseous atmosphere soas to produce a concentration gradient within the gaseous atmospheresuch that the liquor content of the gas passing through said layerincreases in a direction counter-current to the direction of travel ofsaid layer.
 2. A method as claimed in claim 1 in which saidconcentration gradient is produced by effecting flow of said gas incountercurrent relation with the direction of travel of the layer ofparticulate solid material whereby liquor evaporating from the layer ofparticulate solid material in said zone and upstream of said zone issubstantially prevented from passing downstream of said zone.
 3. Amethod of displacing liquor from a particulate solid materialcomprising:forming the particulate solid material into a layer on amovable filter medium; transporting the layer by means of the filtermedium through a washing zone in which the layer is contacted alone thepath of movement thereof with a wash medium while maintaining in aregion over the layer a gaseous atmosphere, the wash medium serving todisplace liquor from the layer and passing, together with the liquor andgas from said atmosphere, through the filter medium; recovering the gasand treating it to eliminate or at least reduce contamination thereof bythe liquor; and reintroducing the treated gas into said atmosphere at alocation such that the gas passing through the layer at a locationdownstream of, or within a downstream section of, the washing zone isless contaminated with liquor than that passing through the layer at alocation upstream of, or within an upstream section of, the washingzone.
 4. A method as claimed in any one of claims 1 to 3 in which thefilter medium moves continuously through said washing zone.
 5. A methodas claimed in any one of claims 1 to 3 in which the washing zonecomprises a series of washing stages arranged in succession along thepath of travel of the filter medium through the washing zone.
 6. Amethod as claimed in claim 5 in which the washing medium is passedthrough the washing stages in counter-current relation with thedirection of movement of the filter medium.
 7. A method as claimed inclaim 3 in which the recovered gas is separated into two streams whichare treated to differing extents such that one stream has a lowercontaminant level than the other, and in which the two streams arereintroduced into said atmosphere at different locations, said onestream being reintroduced at a location downstream of the location atwhich the other stream is reintroduced.
 8. A method as claimed in claim7 in which the recovered gas is subjected to cooling prior to beingsplit into two streams so that contaminants in the vapour phase aresubstantially condensed and thereby separated from the gas.
 9. A methodas claimed in claim 8 in which, following said cooling, the gas is splitinto two streams, said other stream being reintroduced into the gaseousatmosphere without further treatment for contaminant removal.
 10. Amethod as claimed in claim 9 in which said one stream is subjected tofurther treatment to reduce the level of contaminants therein.
 11. Amethod as claimed in any one of claims 1 to 3 and 7 to 10 in which thefilter medium comprises an endless filter belt having a generallyhorizontal upper run on which the layer of solids material is formed.12. A method as claimed in any one of claims 1 to 3 and 7 to 10 in whichthe particulate solid material comprises terephthalic acid crystalsderived from the liquid phase oxidation of p-xylene in a lower aliphaticcarboxylic acid medium.
 13. A method as claimed in claim 12 in which,following said washing treatment, the recovered terephthalic acid isused, without further chemical purification, in the production ofpolyester.
 14. A process for the filtration and washing of terephthalicacid obtained from a reaction vessel as a slurry with an aliphaticcarboxylic acid employed as solvent in the reaction, said processcomprising:(a) contacting the terephthalic acid, initially in the formof said slurry, with a filtration surface acting as a support so as toform the terephthalic acid as a layer on said surface and transportingthe terephthalic acid on said filtration surface through a number ofzones; (b) initially subjecting the slurry to filtration whiletraversing a first zone to remove said solvent through the filtrationsurface and thereby produce a first drained wet deposit; (c) thereafterwashing the first wet deposit while traversing a second zone andeffecting filtration through said filtration surface to separate fromthe terephthalic acid any solvent remaining in the first deposit and thewash liquor used for washing; (d) establishing in said second zoneand/or at least one further zone downstream of the second zone a gaseousatmosphere by means of a gas which passes through the filter medium; andsupplying said gas in such a way that a concentration gradient isproduced in the gas atmosphere such that solvent contamination in thegas passing from said atmosphere through the filter medium decreases inthe direction of travel of the filter medium.
 15. A process as claimedin claim 14 in which the filter medium is configured as a cylindricalloop.
 16. A process as claimed in claim 14 in which the filter medium isconfigured as a cylindrical loop comprising a series of discretesections and in which the gas is employed to effect drying of thefiltered and washed terephthalic acid.
 17. A process for the filtrationand washing of terephthalic acid obtained from a reaction vessel as aslurry with an aliphatic carboxylic acid employed as solvent in thereaction, said process comprising:(a) contacting the terephthalic acid,initially in the form of said slurry, with a filtration surface actingas a support so as to form the terephthalic acid as a layer on saidsurface and transporting the terephthalic acid on said filtrationsurface through first and second zones; (b) initially subjecting theslurry to filtration while traversing said first zone to remove saidsolvent through the filtration surface and thereby produce a firstdrained wet deposit; (c) thereafter subjecting the first wet deposit toa series of washing stages while traversing said second zone andeffecting filtration through said filtration surface to separate fromthe terephthalic acid any solvent remaining in the first deposit and thewash liquor used for washing, said washing stages with the exception ofthe final washing stage being effected using wash liquor obtained from asucceeding stage in countercurrent fashion so that said deposit iscontacted with wash liquor of increasing purity as it progresses throughsaid second zone; (d) pressurising said first and second zones by meansof a gas which passes through the filter medium together with thesolvent and the wash liquor; and (e) supplying said pressurising gas insuch a way that, on the slurry side of the filter surface, the gaspresent in at least the final stage of said second zone is lesscontaminated with said solvent than the gas present in said first zone.18. A process as claimed in any one of claims 14 to 17 in whichfollowing passage through the filter surface the gas is recovered,treated to separate the gas from solvent vapours entrained in the gasflow and re-introduced into the gaseous atmosphere at differentlocations along the path of travel of the filter surface, the gassupplied to the more downstream location or locations being subjected toa higher degree of separation from said solvent.
 19. A system forprocessing a slurry comprising particulate solid material and a liquor,said system comprising:means for filtering the particulate solidmaterial to form a layer thereof on a movable filter medium; a washingzone; means for driving the filter medium so as to transport the layerthrough the washing zone; means for applying wash medium to the layerduring traverse of the washing zone, the wash medium serving to displaceliquor from the layer and passing through the filter medium; and meansfor establishing in said washing zone and/or a zone downstream thereof agaseous atmosphere from which gas passes through the layer and thefilter medium, said atmosphere-establishing means being arranged toproduce a concentration gradient such that the extent to which the gaspassing through said layer and the filter medium is contaminated withsaid liquor decreases in the direction of travel of the filter medium.20. A system as claimed in claim 19 including means for recovering thegas after passage through said layer and treating it to eliminate or atleast reduce contamination thereof by the liquor: andmeans forreintroducing the treated gas into said atmosphere at one or morelocations to produce said concentration gradient.
 21. A system asclaimed in claim 20 in which the means for treating the recovered gasproduces at least two gas streams at least one of which is lesscontaminated with solvent than the others or others and is reintroducedinto said atmosphere at a location downstream of the other gas stream orstreams.
 22. A process for the filtration and washing of terephthalicacid obtained from a reaction vessel as a slurry with an aliphaticcarboxylic acid employed as solvent in the reaction, said processcomprising:(a) contacting the terephthalic acid, initially in the formof said slurry, with a filtration surface acting as a support so as toform the terephthalic acid as a layer on said surface and transportingthe terephthalic acid on said filtration surface through a number ofzones; (b) initially subjecting the slurry to filtration whiletraversing a first zone to remove said solvent through the filtrationsurface and thereby produce a first drained wet deposit; (c) thereafterwashing the first wet deposit while traversing a second zone andeffecting filtration through said filtration surface to separate fromthe terephthalic acid any solvent remaining in the first deposit and thewash liquor used for washing; (d) establishing in said second zoneand/or at least one further zone downstream of the second zone a gaseousatmosphere by means of a gas which passes through the filter medium; andsupplying said gas in such a way that a concentration gradient isproduced in the gas atmosphere such that solvent contamination in thegas passing from said atmosphere through the filter medium decreases inthe direction of travel of the filter medium; (e) separating the gasfrom solvent vapours entrained in the gas flow; and (f) re-introducingthe gas into the gaseous atmosphere at different locations along thepath of travel of the filter surface, the gas supplied to the moredownstream location or locations being subjected to a higher degree ofseparation from said solvent.
 23. A process for the filtration andwashing of terephthalic acid obtained from a reaction vessel as a slurrywith an aliphatic carboxylic acid employed as solvent in the reaction,said process comprising:(a) contacting the terephthalic acid, initiallyin the form of said slurry, with a filtration surface acting as asupport so as to form the terephthalic acid as a layer on said surfaceand transporting the terephthalic acid on said filtration surfacethrough first and second zones; (b) initially subjecting the slurry tofiltration while traversing said first zone to remove said solventthrough the filtration surface and thereby produce a first drained wetdeposit; (c) thereafter subjecting the first wet deposit to a series ofwashing stages while traversing said second zone and effectingfiltration through said filtration surface to separate from theterephthalic acid any solvent remaining in the first deposit and thewash liquor used for washing, said washing stages with the exception ofthe final washing stage being effected using wash liquor from asucceeding stage in countercurrent fashion so that said deposit iscontacted with wash liquor of increasing purity as it progresses throughsaid second zone; (d) pressurising said first and second zones by meansof a gas which passes through the filter medium together with thesolvent and the wash liquor; and (e) supplying said pressurising gas insuch a way that, on the slurry side of the filter surface, the gaspresent in at least the final stage of said second zone is lesscontaminated with said solvent than the gas present in said first zone;(f) separating the gas from solvent vapours entrained in the gas flow;and (g) re-introducing the gas into the gaseous atmosphere at differentlocations along the path of travel of the filter surface, the gassupplied to the more downstream location or locations being subjected toa higher degree of separation from said solvent.
 24. A system forprocessing a slurry comprising a particulate solid material and aliquor, said system comprising:means for filtering the particulate solidmaterial to form a layer thereof on a movable filter medium; a washingzone; means for driving the filter medium so as to transport the layerthrough the washing zone; means for applying wash medium to the layerduring traverse of the washing zone, the wash medium serving to displaceliquor from the layer and passing through the filter medium; and meansfor establishing in said washing zone and/or a zone downstream thereof agaseous atmosphere from which gas passes through the layer and thefilter medium, said atmosphere-establishing means being arranged toproduce a concentration gradient such that the extent to which the gaspassing through said layer and the filter medium is contaminated withsaid liquor decreases in the direction of travel of the filter medium;means for recovering the gas after passage through said layer andtreating it to eliminate or at least reduce contamination thereof by theliquor, wherein the recovered gas produces at least two gas streams, atleast one of which is less contaminated with solvent than the other orothers, and is reintroduced into said atmosphere at a locationdownstream of the other gas stream or streams to produce saidconcentration gradient.